Fig. 1. (A) Predicted fall extractable soil K response surface of Kentucky bluegrass turf fertilized with varying rates of N and K. = 118.73 – 0.158N + 0.495K. R² = 0.978. (B) Predicted spring extractable soil K response surface of Kentucky bluegrass turf fertilized with varying rates of N and K. = 104.10 – 0.167N + 0.442K – 4.654 x 10^–5 NK – 2.105 x 10^–9 NK³ – 1.491 x 10^–11 N²K³. R² = 0.984.